1. Field of the Invention
The invention relates to a device for optically scanning an information plane having tracks, which device comprises an optical system for generating two tracking beams and one main beam, a lens system for focusing the three beams on the record carrier to two tracking spots located at both sides of a track to be scanned and to one main spot on said track, and at least three detection systems for receiving radiation of respectively the two tracking beams and the main beam from the information plan, each detection system being split into at least two detectors. This device can be used in apparatus for writing and reading information on optical record carriers.
2. Description of the Related Art
A scanning device of this type is known from published European Patent Application No. 0,201,603. In the known device a tracking servosystem keeps the main spot on the track to be scanned. This system is controlled by means of a tracking error signal which is derived from the signals of the split detectors. A coarse control and a fine control can be used in the tracking servosystem. The coarse control is realised by moving a slide incorporating the scanning head into the transversal direction, i.e. the radial direction with respect to a round record carrier. The fine control can be realised by displacing an objective lens within the scanning head in the transversal direction. Due to such a displacement of the objective lens the radiation spots formed by the beams in the plane of the detectors will shift with respect to the associated detectors so that an offset is created in the tracking error signal with which the tracking servosystem is controlled.
The aforesaid European Patent Application describes a combination of detector signals producing a tracking error signal which is independent of small transversal displacements of the lens. However, in the case of larger displacements there are serious aberrations in the light path because the lens is used far outside its optical axis. Such displacements occur, for example, at the start of a searching action when the speed of the scanning head is considerably increased so as to proceed to another track on the record carrier. In such a situation the signals supplied by the detectors are often distorted seriously so that errors occur when counting the number of tracks passed during the searching action.